Asked in Pokemon Diamond Pearl and Platinum
How do you get mespirt in Pokemon pearl?
After you catch or beat palkia/dialga, it is possible to go back to the three caves on the three lakes. inside each there is a lake Pokemon. Mesprit is at the first lake by twinleaf town. You wont have a chance to catch him when you first go in because he flies off. Now you have to track him down and randomly battle and catch WARNING he flees from battles instantly so have a Pokemon that knows mean look ar has a special ability that prevents escape. It is level 50 and really hard to catch if you don't have a master ball, and don't kill it, or you wont have another chance to get it.
An easier way to track it down is to get the Marking Map app for the poketch. You get it from the poketch president in Jubilife city after you obtain seven badges. He gives you two other apps before this one though, be sure to get them all.